This invention relates to a magnetic recording medium. More particularly, it relates to a recording medium having a large hysteresis squareness ratio and at the same time having excellent wear resistance, which are owing to an improvement in the orientation characteristics of magnetic particles by virtue of a dispersing agent.
In general, magnetic recording media are produced by coating a substrate made of polyester film or the like with a magnetic coating material prepared from magnetic particles, resins and others, by various coating methods. However, when the magnetic particles have been dispersed nonuniformly in the magnetic coating material, the orientation rate of the magnetic particles in a magnetic recording medium obtained are lowered, thereby resulting in decrease in the hysteresis squareness ratio and causing a lowering of noise characteristics and decrease in reproduction output. For this reason, it has been usually practiced that a dispersing agent such as a surfactant is added to the magnetic coating material in order to enhance the dispersibility of the magnetic particles.
However, most of the dispersing agents known in the art have had problems that, as the amount of the agent to be added increases, there occurs exudation of the dispersing agent on the surface of a magnetic recording medium (i.e. a bleeding phenomenon), adhesion of films of the magnetic recording medium (i.e. a blocking phenomenon) or decrease of wear resistance due to the softening of coated films.
For instance, according to a finding previously made by the present inventors, employment of an ester of phosphoric acid with monoalkyl- or monoalkylphenylpolyethylene glycol as a dispersing agent for the magnetic particles was useful for improvement in the dispersibility of the magnetic particles contained in a magnetic coating material, thereby remarkably increasing the hysteressis squarencess ratio of a magnetic recording medium. Such a dispersing agent, however, was defective in that, when it is added in a greater amount, there occurred on coated films undesirable phenomena such as bleeding, blocking and softening, and the wear resistance of the coated film drastically decreased.
Accordingly, it has been desired to develop a magnetic recording medium having overcome the defects mentioned above, and large in both the hysteresis squareness ratio and the wear resistance.